1. Field of the Invention
The invention relates to waterway channel maintenance. More specifically, the invention relates to channel dredging, particularly channel side dredging.
2. Description of the Prior Art
Shoaling and the buildup of silt and mud are common problems around docks, ports and channels, and frequent dredging is required to maintain sufficient depth of water for use by ships, barge and other waterborne vehicles. Many types of dredge equipment have been developed, including cutter-head dredges, bucket or shovel dredges, and side trailing hopper dredges. These dredge systems use mechanical manipulation of the dredged material, either by pumping or scooping, to transfer the material away from the area being dredged, e.g. a dock or channel side. An example of a dredging system using pumping is disclosed in U.S. Pat. No. 4,352,251. This reference discloses a hand operated suction dredge head and a hydraulic submersible pump assembly. Water and sludge is driven through a conduit and pump and is discharged to a location remove from the dredge head. While these systems are somewhat effective in dredging level surfaces, the angularity of channel sidewalls can present challenges to the operation of certain types of these dredges.
Another problem present with prior dredge systems is that they are destructive to bottom dwelling creatures because of the mechanical action of the dredging. Also associated with mechanical dredging systems is the problem of finding a location for the dredged material after it has been removed. Very often the ship from which the dredging is taking place must act as a temporary storage area for the removed material. This necessitates a larger ship size than otherwise needed, thereby requiring more crew and a higher United States Coast Guard license rating to operate the vessel. An additional special problem with systems that transfer material by pumping is that the slurry dredged up and pumped through the pumps is very abrasive. This abrasiveness causes the pumps to wear at a higher rate than pumps just pumping water.
A more recent system, referred to as water injection dredging (WID), has shown advantages in dredging channel side buildup. Generally, WID involves injecting water into the material, liquefying it and causing it to flow under its own weight to deeper adjacent water. An example of WID is disclosed in U.S. Pat. No. 4,604,000. While this technique can be useful for higher level side dredging, it is largely ineffective for bottom dredging where there is no lower runoff level.
Another problem associated with both WID and mechanical dredging systems is that they produce a layer of "fluff" or low density mixture of bottom material and water just above the dredged bottom. When survey equipment tries to measure the depth of the bottom the depth recording equipment records the fluff layer rather than the true dredged bottom.